Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $51.84   12.362%   $58.2485   $58.25   $58.25   $58.20  
  Purchase #2 $50.37   13.265%   $57.0516   $57.05   $57.05   $57.10  
  Purchase #3 $32.43   12.541%   $36.497   $36.50   $36.50   $36.50  
  Purchase #4 $114.84   7.590%   $123.5564   $123.56   $123.55   $123.60  
  Purchase #5 $152.34   9.430%   $166.7057   $166.71   $166.70   $166.70  
  Purchase #6 $193.27   5.185%   $203.291   $203.29   $203.30   $203.30  
  Purchase #7 $134.90   13.852%   $153.5863   $153.59   $153.60   $153.60  
  7 purchase totals = $729.99       $798.9365   $798.95   $798.95   $799.00